Para-xylene is a valuable chemical feedstock, which may be derived from mixtures of C8 aromatics separated from such raw materials as petroleum naphthas, particularly reformates, usually by selective solvent extraction. The C8 aromatic fractions from these sources vary quite widely in composition but will usually comprise 10 to 32 wt % ethylbenzene (EB) with the balance, xylenes, being divided between approximately 50 wt % of the meta isomer and 25 wt % each of the para and ortho isomers. Of these isomers, para-xylene is by far the most important.
Individual isomer products may be separated from the naturally occurring C8 aromatic mixtures by appropriate physical methods. Ethylbenzene may be separated by fractional distillation, although this is a costly operation. Ortho-xylene may be separated by fractional distillation, and is so produced commercially. Para-xylene may be separated from the mixed isomers by fractional crystallization, selective adsorption (e.g., the Parex process), or membrane separation.
As commercial use of para-xylene has increased, combining physical separation with chemical isomerization of the other xylene isomers to increase the yield of the desired para-isomer has become increasingly important. However, since the boiling point of ethylbenzene is very close to those of para-xylene and meta-xylene, complete removal of ethylbenzene from the C8 aromatic feed by distillation is impractical. Hence, an important feature of any commercial xylene isomerization process is the ability to convert ethylbenzene in the feed to useful by-products while simultaneously minimizing any conversion of xylenes to other compounds.
One commercially successful xylene isomerization process is described in U.S. Pat. No. 4,899,011 in which a C8 aromatic feed, which has been depleted in its para-xylene content, is contacted with a two component catalyst system. The first catalyst component selectively converts the ethylbenzene by deethylation, while the second component selectively isomerizes the xylenes to increase the para-xylene content to a value at or approaching the thermal equilibrium value. The first catalyst component comprises a Constraint Index 1 to 12 zeolite, which has an ortho-xylene sorption time of greater than 50 minutes based on its capacity to sorb 30% of the equilibrium capacity of ortho-xylene at 120° C. and an ortho-xylene partial pressure of 4.5±0.8 mm of mercury, whereas the second component comprises a Constraint Index 1 to 12 zeolite which has an ortho-xylene sorption time of less than 10 minutes under the same conditions. In one preferred embodiment, the first catalyst component is ZSM-5 having a crystal size of at least 1 micron and the second catalyst component is ZSM-5 having a crystal size of 0.02 to 0.05 micron. Each catalyst component also contains a hydrogenation metal, preferably a noble metal such as platinum or palladium.
An improvement over the process of U.S. Pat. No. 4,899,011 is described in U.S. Pat. No. 5,689,027 in which the first catalyst component in the two component system is pre-selectivated by coking, or more preferably by deposition of a surface coating of silica, to increase its ortho-xylene sorption time to greater than 1200 minutes under the same test conditions as cited in the '011 patent. Using such a system it is found that high ethylbenzene conversion rates can be achieved with significantly lower xylene losses than obtained with the process of the '011 patent. Again, the catalyst components employed in the process of the '027 patent include a hydrogenation metal, such as Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, Pt, Mn, Re, Cu, Ag, Au, Cr, Mo, W, Ga, In and Bi, preferably Pt.
In addition to potential xylene loss especially at high ethylbenzene conversion, the two component xylene isomerization/ethylbenzene dealkylation processes described above pose a number of additional concerns. For example, it is important that the hydrogenation metal, particularly that associated with the first catalyst component, provides excellent selectivity for ethylene saturation without saturating benzene such that the benzene fraction after distillation from the final product meets the 99.9 wt % benzene purity specification. In addition, it is important that the catalyst components are resistant to sulfur poisoning and the CO-induced metal migration that can occur on start-up. Developing a catalyst system and operating process that achieves all these requirements remains an area of ongoing research.